Method of fabricating mercury-wetted switching devices



Sept. 8, 1964 R. E. PERKINS 3,147,538

METHOD OF FABRICATING MERCURY-WETTED SWITCHING DEVICES Filed May 16, 1961 .P RKI RICHARD EE E QS ATTO EY United States Patent 3,147,538 METHOD OF FABRICATING MERCURY-WETTED SWITCHING DEVICES Richard E. Perkins, Ipswich, Mass, assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed May 16, 1961, Ser. No. 110,458 4 Claims. (Cl. 29-1555) This invention relates to the manufacture of mercurywetted relay switches.

Mercury-wetted contact relays are designed to give over a billion reliable, high speed operations with complete freedom from maintenance problems. They are adaptable for many applications of high speed switching, such as required in computers and data processing equipment. The relay comprises a hermetically sealed glass envelope containing at least a pair of contacts and an armature, the armature being positioned so as to be movable under the influence of a magnetic field set up by a surrounding coil, and at least one of said contacts being movable by said armature. A steel case having a plurality of protruding contact pins encloses the entire assembly of the envelope and the surrounding coil.

. Such mercury-wetted relay switches are known to the art and methods for preparing them have previously been described. In general, the envelope of such relays contains a reservoir of mercury which rises by capillary action and Wets the enclosed metal parts, thereby presenting an everchanging surface at the contact points and materially reducing wear.

I have found that for the mercury to wet the contact points of the relay properly, it is essential that the mercury wet all metal parts in the envelope. Such wetting of all parts is often rather ditficult to obtain however, since impurities present on the metal surfaces apparently impede this action. Although extreme care may be used in preparing the relay and choosing the materials, impurities may develop due to manipulative steps in the fabrication. In particular, heat is required to seal the various contacts to the glass and to seal the envelope, but this heat also tends to cause the metal parts to oxidize. The oxides thus produced are quite troublesome, since they prevent thorough mercury wetting.

My invention concerns a method of fabricating a merc ury-wetted relay including the step of cleaning impurities from the metal parts after insertion of a quantity of mercury into the relay, but before final sealing of the envelope. Furthermore, concurrently with the cleaning step, metal surfaces are conditioned by what appears to be a pre-wetting, so that in a subsequent step, they may be thoroughly coated with the mercury. In particular, the simultaneous cleaning and pre-wetting step comprises ionizing some of the mercury from the mercury reservoir while the envelope is continually exhausted and under a vacuum. Such ionization may be produced by placing the electrical discharge from a high-voltage, discharge device, for example a high leakage reactance transformer or a spark coil, near the exterior walls of the glass envelope. To obtain eificient cleaning, the electrical discharge should be maintained for at least a suflicient time to produce a rich mercury vapor glow throughout the relay although longer periods are preferred. Apparently physical contact of the mercury ions upon the metal surface erodes impurities, thereby cleaning the surfaces, and a thin, generally invisible layer of mercury is deposited 3,147,538 Patented Sept. 8, 1964 which prepares the parts for a final mercury wetting.

Accordingly, an object of my invention is to clean metal parts of devices using mercury for contact surfaces, such as found in mercury-wetted relay switches.

A feature of my invention is cleaning metal parts enclosed in an envelope through the use of mercury ions while the envelope is under vacuum.

Another feature of my invention is that metal surfaces in an envelope are cleaned with mercury ions which are produced when a mercury reservoir inside of the envelope is excited by a high-voltage, high-frequency discharge device.

An advantage of my invention is that metal parts of the relay are cleaned after all glass sealing operations and this cleaning takes place with materials necessarily present in the envelope. Furthermore, such cleaning prewets metal surfaces for subsequent mercury wetting operations.

Other objects, features and advantages will become apparent to those skilled in the art upon reading the following specification, when taken in conjunction with the accompanying drawing, which illustrates a cross-sectional view of a mercury-wetted relay having an exhaust tube inserted in a vacuum pump and fill system and the external surfaces of the envelope being contacted with the electrical discharge from a high-voltage, high-frequency dis charge device.

The mercury-wetted relay shown in the drawing is inverted from its normal operating position, but the relay will be described with reference to operating positions rather than fabrication positions.

Referring now to the drawing, the relay comprises an outer casing, a glass envelope 1 that may be prepared, for convenience, of lead glass having a low melting point. Sealed in a conventional manner through the upper end of the envelope 1 is a first seal assembly which comprises a pair of front leads 3 (only one of which is shown in the drawing) and a pair of back leads 5 (only one of which is shown in the drawing). Attached to each of the front and back leads, 3 and 5 respectively, are small platinum contacts 7 and 9. A swinger assembly is sealed with conventional metal to glass sealing methods through the lower end of the envelope 1. The sub-assembly components of the swinger assembly are mounted on the inner end of an exhaust tube 11 and comprise a pole piece subassembly and an armature sub-assembly. Each of these sub-assemblies is attached to the same inner end of the exhaust tube, but on opposite sides thereof. In the pole piece sub-assembly, a U-shaped, large wick 15 forms the attachment between pole piece 12 and shim 14. Attached to the opposite side of the exhaust tube 11 is the armature sub-assembly which swings back and forth on spring 20 in response to the influence of a magnetic field set up by a surrounding coil (not shown). One end of a flat, generally rectangular spring 20 is attached to exhaust tube 11 and the other end is attached to U-shaped small wick 19. Armature 17 is attached to the small wick 19 which serves to space the armature 17 from spring 20. Aflixed to the lower portion of armature 17 is a platinum butterfly contact 6. In a fabricated device, butterfly contact 6 reciprocates on spring 20 between platinum contacts 7 and 9 in response to the magnetic field set up by a surrounding coil (not shown) but is usually mechanically biased against platinum contact 9.

After the swinger and first seal assemblies are sealed through opposite ends of the envelope 1, a quantity of mercury for example to 1 cc., and preferably to cc., is inserted into the envelope with a hypodermic needle through the exhaust tube Ill to form a reservoir 21. The mercury inserted should be at least sufficient to coat all metal parts, but should occupy less than one third of the volume of the envelope. The externally protruding end of exhaust tube 11 is then inserted into a pump and fill system 30 and a vacuum pump (not shown) connected to line 2 started. The preferred vacuum for the cleaning operation is below about 200 microns, since the mercury is most easily ionized with such pressures, however in some cases this pressure may be varied so long as there is continual evacuation of the envelope. After the pressure within the relay is reduced to below about 200 microns, the electrical discharge from a high voltage, high frequency discharge device 8, which may be for example the illustrated Tesla coil, is applied to the outside of the envelope 1.

Generally, about 8000 volts should be developed to ionize the mercury, but this requirement may vary depending chiefly upon the pressure within the envelope. The Telsa coil, for example the unit prepared by the Ecco High Frequency Corp. Model G-4, has a particular advantage for our purposes, since the current is of the order of microamperes and the frequency in the order of one megacycle per second, thus minimizing any danger to the operator. Even with an ordinary high leakage transformer, operating at 60 cycles per second, the current is small, generally not more than about 5 milliamperes.

After applying this high voltage to the mercury reservoir 21 for about 4 or 5 seconds, the mercury is warmed and some of the mercury vaporized. The discharge device 8 causes the mercury vapor to ionize and the mercury ions will, in turn, clean the surfaces of the metal parts. The length of time for keeping the mercury ionized can vary from 15 seconds to 1 minute; however, such limitations are not critical so long as a rich mercury vapor glow is established throughout the envelope 1. Care must be taken not to concentrate the electrical discharge from the discharge device 8 on any one spot of the envelope 1 for too long a period, since such contact might burn through the glass and ruin the relay. Thus, the application of the discharge device 8 to the exterior surfaces of the envelope 1 involves first directing the spark toward the mercury reservoir 21 until the mercury is warmed and vaporized and the moving it along the length of the envelope 1 to ionize the vapor.

The physical contact of the cloud of mercury ions upon the metal surfaces appears to remove any contaminating oxide coating. Because of the continued evacuation of the envelope 1 during the excitation of the mercury vapor, these oxide contaminants are continually removed from the envelope 1 through the exhaust tube 11 together with some mercury ions. Since large excesses of mercury have been originally placed in the envelope 1, there is no need to insert supplemental mercury after the cleaning to compensate for mercury ions removed during evacuation.

The effect of the above-described mercury-ion cleaning method is not only to cleanse the metal surfaces of adhering oxides, but also it is to prewet them with mercury. It would appear that when a rich glow of mercury vapor is established in the envelope, a blanket of memory ions is laid down upon the metal surfaces so that upon cooling, a thin, generally invisible layer of mercury will form. This layer forms not only on the larger and exposed surfaces in the envelope, but also it forms in the small interstices between the various elements. Such prewetting in the interstices is quite advantageous, since they are quite difiicult to coat with mercury when using ordinary procedures.

After cleaning and prewetting the metal parts, the ionization of the mercury is discontinued and the vacuum pump turned off. For most relays, hydrogen gas is used to fill the envelope although many other suitable gases may be added. Generally, pressures in the range of 250 lbs. per sq. inch are preferred, however such pressures may be varied to suit the individual relay. After adding the hydrogen gas through line 28, the exhaust tube 11 is sealed off at a point closely adjacent the envelope 1. In this manner, a fully assembled relay is prepared containing a reservoir of mercury and filled with hydrogen gas. All of the metal surfaces in the envelope are cleaned and prewet, however the mercury has yet to form the thick amalgam coating on the metal parts, as required for efficient and long use of the relay.

In order to form an amalgam and coat all of the metal parts with the mercury from the reservoir 21, the sealed envelope is heated in an oven at temperatures of 200 to 400 C. for 10 to 20 minutes. Proper coating of all metal parts is insured by rotating the relays about their axes during the heating operation.

My method for fabricating a mercury-wetted relay, including the step of cleaning the metal parts, requires no additional reactants and the step may easily be integrated in existing process lines. Thus, it is quite eflicient and economical.

It is apparent that changes and modifications may be made in the instant invention, but it is my intent, however, to be limited only by the scope of the appended claims.

As my invention I claim:

1. In the method of fabricating a mercury-wetted, electrical switching device housed in a glass envelope, said device having exposed metal surfaces, wherein said device has at least one electrical contact sealed through an end of said envelope and wherein another electrical contact is affixed to a metal exhaust tube sealed through the other end of said envelope, the steps which comprise: adding mercury to said envelope through said exhaust tube; evacuating gases present in said envelope through said exhaust tube thereby reducing the pressure in said envelope; ionizing a portion of said mercury while continuing the evacuation and hence producing a visible glow, thereby cleaning and pre-wetting said exposed metal surfaces in said envelope; filling said envelope with a fill gas and sealing said exhaust tube.

2. In the method of fabricating a mercury-wetted, electrical switching device housted in a glass envelope, said device having exposed metal surfaces, wherein said device has at least one electrical contact sealed through an end of said envelope and wherein another electrical contact is affixed to a metal exhaust tube sealed through the other end of said envelope, the steps which comprise: adding mercury to said envelope through said exhaust tube; evacuating gases present in said envelope through said exhaust tube, thereby reducing the pressure in said envelope; ionizing. a portion of said mercury while continuing the evacuation and hence producing a visible glow by exposing the external surfaces of said envelope to a high-voltage electric discharge, thereby cleaning and prewetting said exposed metal surfaces in said envelope; filling said envelope with a fill gas and sealing said exhaust tube.

3. In the method of fabricating a mercury-wetted, electrical switching device housed in a glass envelope, wherein said device has at least one stationary electrical contact sealed to an end of said envelope and wherein at least one reciprocable electrical contact is afiixed to a metal exhaust tube sealed through the other end of said envelope, said device having exposed metal surfaces, the steps which comprise: adding mercury tosaid envelope through said exhaust tube; reducing the pressure in said envelope to below 200 microns by evacuating gases present therein through said exhaust tube; ionizing a portion of said mercury while continuing the evacuation and hence producing a visible glow, thereby cleaning and pre-wetting said exposed metal surfaces in said envelope; filling said envelope with a fill gas and sealing said exhaust tube.

4. In the method of fabricating a mercury-wetted, electrical switching device housed in a glass envelope, wherein said device has at least one stationary electrical contact sealed to an end of said envelope and wherein at least one reciprocable electrical contact is aflixed to a metal exhaust tube sealed through the other end of said envelope, said device having exposed metal surfaces, the steps which comprise: adding mercury to the said envelope through said exhaust tube; reducing the pressure in said envelope to below 200 microns by evacuating gases present in said envelope through said exhaust tube; exposing the external surfaces of said envelope to a highvoltage electric discharge, thereby ionizing a portion of said mercury whereby a visible glow is produced and thus, cleaning and prewetting said exposed metal surfaces in said envelope; filling said envelope with a fill gas and sealing said exhaust tube.

References Cited in the file of this patent UNITED STATES PATENTS Pollard July 20, 1946 Burton Jan. 18, 1949 

1. IN THE METHOD OF FABRICATING A MERCURY-WETTED, ELECTRICAL SWITCHING DEVICE HOUSED IN A GLASS ENVELOPE, SAID DEVICE HAVING EXPOSED METAL SURFACES, WHEREIN SAID DEVICE HAS AT LEAST ONE ELECTRICAL CONTACT SEALED THROUGH AN END OF SAID ENVELOPE AND WHEREIN ANOTHER ELECTRICAL CONTACT IS AFFIXED TO A METAL EXHAUST TUBE SEALED THROUGH THE OTHER END OF SAID ENVELOPE, THE STEPS WHICH COMPRISE: ADDING MERCURY TO SAID ENVELOPE THROUGH SAID EXHAUST TUBE; EVACUATING GASES PRESENT IN SAID ENVELOPE THROUGH SAID EXHAUST TUBE THEREBY REDUCING THE PRESSURE IN SAID ENVELOPE; IONIZING A PORTION OF SAID MERCURY WHILE CONTINUING THE EVACUATION AND HENCE PRODUCING A VISIBLE GLOW, THEREBY CLEANING AND PRE-WETTING SAID EXPOSED METAL SURFACES IN SAID ENVELOPE; FILLING SAID ENVELOPE WITH A FILL GAS AND SEALING SAID EXHAUST TUBE. 